Composite Video Signal (1)

8/3/2019 Composite Video Signal (1)

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3. COMPOSITE VIDEO

SIGNAL

Prepared by

Sam Kollannore U.

Lecturer, Department of ElectronicsM.E.S.College, Marampally, Aluva-7


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COMPOSITE VIDEO SIGNAL

Consist of : Camera signal - corresponding to the desired picture

information

Blanking pulses to make the retrace invisible

Synchronizing pulses to synchronize the transmitter and

receiver scanning-horizontal sync pulse

-vertical sync pulse

-their amplitudes are kept same

-but their duration are different

-needed consecutively and not simultaneouslywith the picture signal so sent on a time divisionbasis


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Composite Video Signal contd


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Composite video signal contd

Video signal varies between certain limits

Peak white level: 10 to 12.5%

Black level : 72%

Blanking level : Sync pulses added - 75%

level

Pedestal : difference between black level

and blanking level tend to merge

Pedestal height : distance between thepedestal level and the dc level indicates

the average brightness

Picture information : 10% - 75%

Darker the picture higher will be the voltage within those limits


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DC component of the video signal

Average value or dc component corresponding

to the average brightness of the scene

Average brightness can change only fromframe to frame and not from line to line

Low pedestal height scene darker

Larger pedestal height higher averagebrightness


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blanking pulses . . .

Make the retrace lines invisible

by raising the signal amplitude

slightly above the black level

(75%)

Repetition rate of horizontal

blanking pulse = scanning freq.

= 15625Hz

Freq of vertical blanking pulse

= field scanning freq. = 50 Hz


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Sync Pulse and Video Signal Amplitude Ratio

P/S RATIO = 10/4

Justification: If the picture signal amplitude is at the expense

of sync pulses when S/N ratio at the receiver falls,sync pulse amplitude becomes insufficient to keepthe picture locked

If the sync pulse amplitude is at the expense of

the picture signal, then the raster remains lockedbut the amplitude of the picture content will be toolow

P/S ratio of 10/4 represents the most efficient useof TV system


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horizontal Sync details . . .


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horizontal sync details contd

Total line period = 64S

Line blanking period = 12S

Differential leading edges are used for synchronizing horizontalscanning oscillator

Divided into three sections :

front porch: 1.5S - allows the receiver video to settle down

line sync : 4.7 S - for blanking the flyback/retrace

- blacker than the black

back porch: 5.8S - time for the horizontal time base circuit toreverse the direction of current for scanning the next line

- same amplitude as that of blanking level ; used by AGC

circuits at the receiver to develop true AGC voltage


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Vertical Sync details


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Vertical Sync details contd.

Added after each fields

Complex in nature

Vertical sync period = 2.5 to 3 times the horizontal line

period In 625 line system: 2.5 64 = 160S

Commence at the end of first half of 313th line (end offirst field) and terminates at the end of 315th line

Similarly after an exact interval of 20mS (one fieldperiod), the next sync pulse occupies the line numbers1st, 2nd and first half of 3rd .


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Vertical sync details contd Horizontal sync information is extracted from the sync pulse

train by differentiation i.e. Passing the pulse train through anHPF leading edges are used to synchronize the horizontalscanning oscillator

Furthermore, receivers often use monostable multivibratorsto generate horizontal scan, and so a pulse is required to

initiate each and every cycle of the horizontal oscillator inthe receiver.


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Shortcomings and its solution1. Horizontal sync pulses are available both during the active

and blanked line periods but there are no sync pulses(leading edges) available during the 2.5 line vertical syncperiod horizontal sweep oscillator would tend to step outof synchronism during each vertical sync period

The situation after an odd field is even worse

-since it begins at midway-leading edge of the vertical sync pulse comes at thewrong time to provide synchronism for the horizontaloscillator

Therefore five narrow slots (4.7S width) are cut in thevertical sync pulse at intervals of 32S risingedges are used to trigger horizontal oscillator.

This insertion of short pulses : called notching of serration ofthe broad field pulses


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Shortcomings and its solution .contdnotching of serration of the broad field pulses

notching of serration of the broad field pulses


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2. It is seen that the synchronization of the vertical sweep oscillator in

the receiver is obtained from vertical sync pulses by integrator (LPF) Voltage built across the capacitor of the LPF corresponding to the

sync pulse trains of both the fields is shown in fig.

Shortcomings and its solution .contd


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Each horizontal pulse cause a slight rise in voltage across the capacitor,but this is reduced to zero by the time the next pulse arrives (charging

period=4.7S and discharging period = 59.3S)

But during broad serrated region, capacitor has more time to chargeand only 4.7S to discharge

Situation is different for the beginning of the 2nd field-here the lasthorizontal pulse corresponding to the beginning of the 313th line isseparated from the first vertical pulse by only half-a-line.

Therefore the voltage developed a/c the vertical filter will not haveenough time to reach zero before the arrival of the 1st vertical pulse

Hence the voltage developed a/c the o/p filter is some what higher ateach instant as compared to the voltage developed at the beginning ofthe 1st field (shown as dotted chain)

i.e. Oscillator get triggered a fraction of a second early as compared tothe first field - upset the desired interlacing sequence

Equalizing pulses are used to solve this problem

Shortcomings and its solution .contd


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Equalizing pulses Solves the shortcomings occurring on account of half line discrepancy

Five narrow pulses of 2.5 line period are added on either side of the

vertical sync pulses : known as pre-equalizing and post-equalizing

pulses

The effect of these pulses is to shift the half line discrepancy away

from both the beginning and end of the vertical sync pulses


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Equalizing pulses . . . contd

Pre-equalizing pulses:- 2.3S duration

- result in the discharge of the capacitor to zero voltage

in both the fields

Post-equalizing pulses: necessary for a fast discharge of thecapacitor to ensure triggering of the vertical oscillator atproper time

With the insertion of equalizing pulses:- the voltage rise and fall profile is the same for both the

field sequences

- the vertical oscillator is triggered at the proper instants.

i.e. exactly at an interval of 1/50th of a second.

Documents

Composite Video Signal (1)